The present invention relates generally to xe2x80x9claser printersxe2x80x9d and more specifically to apparatus and circuitry for dynamically aligning the laser beam of printers which use MEMS (micro-electric mechanical systems) type mirrors (such as dual axis torsional hinge mirrors) to provide bi-directional raster type scanning across a moving photosensitive medium, such as a drum. A first set of torsional hinges is used for providing the raster scan of a controlled resonant frequency about a first axis.
Alignment of the printer scan can be continuously monitored and dynamic adjustments made during printer operation by adjusting the DC current of its drive coils to position the center of the laser sweep on the photosensitive medium.
Rotating polygon scanning mirrors are typically used in laser printers to provide a xe2x80x9crasterxe2x80x9d scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous printed image line. Alignment of the optical components comprising this type of printer is accomplished during the manufacturing process. Any subsequent realignment requires slow and difficult positioning and securing in place the various optical components. Thus, there is no alignment or realignment available for temporary misalignment such as might occur as a result of temperature changes.
Prior art efforts to use a single and typically much less expensive flat mirror with a single reflective surface, such as a resonant mirror, to provide a scanning beam unfortunately require significant compromise in performance. For example, the rotating photosensitive drum and the scanning mirror generating the beam sweep or scan can be synchronized as the xe2x80x9cresonantxe2x80x9d mirror first pivots or rotates in one direction such that a first printed image line on the medium is at right angles or orthogonal with the movement of the photosensitive medium. Unfortunately, however, the return sweep will traverse a trajectory on the moving photosensitive drum which will be at an unacceptable angle with the first printed image line resulting from the previous sweep. Consequently, if such a single reflecting surface resonant mirror is to be used, it is necessary to interrupt the modulation of the reflected light beam and wait for the mirror to complete the return sweep or cycle, and then again start scanning in the original direction. This requirement of only using one of the sweep directions of the mirror, of course, reduces the print speed and requires expensive and sophisticated synchronization between the mirror and the rotating drum.
Texas Instruments presently manufactures a two-axis analog mirror MEMS device fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100-115 microns. The layout consists of a mirror normally having a size of about 3.8 millimeters by 3.2 millimeters supported on a gimbal frame by two silicon torsional hinges. The mirror may be of any desired shape, although an oval shape is often preferable. As an example, an elongated oval shaped mirror having a long axis of about 5.5 millimeters and a short axis of about 1.2 millimeters has been found to be especially suitable. The gimbal frame is attached to a support frame by another set of torsional hinges. A similar single axis mirror device may be fabricated by eliminating the gimbal frame and hinging the mirror directly to the support frame. A first pair of drive coils controls the movement of the mirror about one of the two axes and a second pair of drive coils controls the movement of the mirror about the second axis. This Texas Instruments manufactured mirror is particularly suitable for use with a laser printer by using one set of coils to generate a resonant sweep of the mirror at a selected frequency. The other set of coils provides slight movement to assure printed image lines are at right angles to the printed page.
However, as was true with rotating polygon mirror laser printers, alignment of the optical components is very important if quality printing is to be achieved.
Therefore, there would be a significant advantage and improvement to laser printers if the optical alignment of the components could be continuously monitored and dynamically adjusted when misalignment occurs for substantially any reason including temperature variations, shock, vibration or even component work.
The problems mentioned above are addressed by the present invention which, according to one embodiment, provides a scanning mirror apparatus suitable for use as the means of generating a sweeping or scanning beam of light across a moving photosensitive medium, such as a rotating drum, in a laser printer. The mirror apparatus comprises a mirror device including a reflective surface portion positioned to intercept the beam of light from a light source. The reflective surface of the mirror device is supported by a first hinge arrangement, such as torsional hinges, for pivoting around a first axis and is further supported by a second hinge arrangement for pivoting about a second axis substantially orthogonal to the first axis. Thus, pivoting of the mirror device about the first axis results in a beam of light reflected from the reflective surface sweeping along a first plane and pivoting of the device about the second axis results in the reflective light beam moving in a direction which is substantially orthogonal to the first plane. The mirror apparatus also includes a first driver for alternately causing pivoting in one direction about the first axis and then the opposite direction to provide a beam sweep or scanning across a moving photosensitive medium. The moving photosensitive medium is located to receive a modulated image of the reflected light beam as it sweeps a trace across the drum or moving medium between a first edge and a second edge in one direction across the medium as the mirror device pivots about the first axis. The photosensitive medium rotates or moves in a direction such that sequential images or traces are spaced from each other. Therefore, to avoid the image line being printed at an angle on the page, there is also included a second driver for pivoting the mirror about the second axis such that an image line of the sweeping beam received on the photosensitive medium or drum is maintained substantially orthogonal to the movement of the photosensitive medium. A pair of light sensors are located to intercept the sweeping light beam at the beginning and end of both the forward and the reverse sweeps of a sweep cycle. Computational circuitry receives the signals from the light sensors. The signals are then used to determine the center of the beam sweep. The center of the sweep is then aligned or adjusted with respect to the photosensitive medium by changing the DC bias level applied to the first drive coil.